Method for the treatment of aramid material and fiber, yarn, and fabric made thereby

ABSTRACT

This invention relates to a method for providing a dyed meta-aramid material, wherein said method comprises at least a step of pretreatment of the material and a subsequent step of dyeing the material, characterized in that the step of pretreatment is carried out under closed conditions, in the presence of an aqueous medium, and at a target temperature of 80 to 130° C. This invention also relates to fibers, yarns, and fabrics obtained from this method.

The present invention includes the technical field of treatment of dyeable meta-aramid materials, for example fibers known under the trade name NOMEX®, and yarns and fabrics made of such fibers for use in the fabrication of protective articles such as fire protection clothing.

U.S. Pat. No. 7,156,883 to Lovasic & Maini relates to a blend of fibers for use in protective clothing, a lightweight fabric made from such blend, protective articles made from the blend or fabric, and methods for making the fabric. The protective fabrics and articles of this invention have the unique combination of being comfortable, being highly effective against electrical arcs and flash fire hazards and having a pleasing appearance. Specifically, these fabrics can be processed to give the look and feel similar to conventional clothing fabrics such as denim fabrics. U.S. Pat. No. 7,156,883 discloses a fiber blend that comprises amorphous meta-aramid fiber, crystallized meta-aramid fiber, and flame retardant cellulosic fiber. In one embodiment relates to a fabric for protective apparel made from a first yarn comprising amorphous meta-aramid fiber and flame retardant cellulosic fiber and a second yarn comprising crystallized meta-aramid fiber and flame retardant cellulosic fiber. Preferably, the first and second yarns are present transverse each other in the fabric.

From U.S. Pat. No. 7,156,883 it is known that aramid fabrics are more difficult to dye than traditional apparel fabrics, and the percent crystallinity of aramid fiber dramatically affects the degree to which the fiber may be dyed. The higher the crystallinity of the aramid fiber, the harder it is to dye. It is especially difficult to give such aramid fabrics the general appearance of a cotton denim fabric due to the differences in aramid fiber crystallinity. The simple addition of cotton, by blending cotton fiber with the meta-aramid fiber, does not provide a suitable solution to this problem. Cotton must be chemically treated to make it flame retardant. This is done in fabric form, which stiffens and reduces the suppleness of the fabric. This makes any protective apparel made from this fabric less comfortable than apparel made from the untreated fabric.

US Pat. Publication 2007/0006400 to Brown discloses yarns and fabrics, principally covering polyesters, formed from such yarns incorporating an arrangement of discrete zones of variable heat treat history thereby imparting differential dye affinity and structural character at discrete zones along the yarn length. The differential dye affinity permits variable shading along the yarn length when the yarn is subjected to a dye bath. The different zones may also exhibit selective shrinking during post-formation heat setting. More specifically, after exiting a heat treating unit, yarn sheet enters a yarn inspection system to detect any breakage of the individual yarns. The yarn may then be wound by a winder into packages. The packages of yarn may thereafter be formed into a fabric such as by weaving, knitting or the like that is subjected to dyeing and finishing treatments to yield a final textile structure. As indicated, the yarns have a variable crystalline character along their length. Such variable crystalline character is believed to yield variable dye affinity along the length of the yarn. Thus, after dyeing and heat treatment, a final textile construction incorporating the yarn is characterized by variable coloration across its surface.

In a normal production process, it is common that the yarns/fibers used to form a fabric originate from different batches of products. Even if the process to obtain such batches of products is normally identical and carried out under the same conditions (for example in terms of temperatures and pressures), it cannot be ensured that the yarns obtained from such batches have identical end properties. For example, spun yarns made from short fibers or stretch broken precursors would tend to exhibit more variability including orientation characteristics that may affect measurements of their properties. Since the crystallinity affects the degree to which the fibers may be dyed, it is instrumental that this parameter remains constant from production batch to production batch, to ensure uniform dyeing over industrial manufacturing processes. In fact, variations in the crystallinity index of the different production batches would require ongoing modification of the dye recipes which cannot be accepted from an industrial point of view.

US Pat. Publication 2008/0295232 by Truesdale discloses systems and methods for dyeing inherently flame resistant fibers, and particularly aramid fibers, without the use of accelerants or carriers. Fabrics made from aramid fibers or blends thereof are immersed in an aqueous dye bath that includes at least one dye and at least one acid component. The temperature of the dye bath is increased from room temperature to a suitable temperature (e.g., between approximately 285° F. to 400° F.) capable of rendering the aramid fibers less crystalline so that the fibers can accept the dye. In this way, suitable color yields may be obtained without the use of accelerants or carriers as have been required in the past. Fabrics dyed in accordance with the method described in this prior art may subsequently be used to make a variety of protective garments, including, but not limited to, coveralls, jumpsuits, shirts, jackets, vests, and trousers, for protecting the wearer against thermal hazards such as electrical arcs and flames.

Hence, the market is confronted with irreversible and reoccurring streakiness in the production of dyed fabrics made of such aramid precursor fibers: these dyed fabrics appear as marked with stripes or linear discolorations and have an uneven character or appearance. These problems are due to the inherent variations of the morphological structure of said fibers. Unfortunately these streakiness problems show up only after the fabric has undergone the final dye processing step at the end of the production process. A visual or quality inspection before the dyeing process is not possible since the aramid material, for example a fabric, does not present a visual specific appearance that would allow anticipating this potential dyeing problem.

It is sometimes possible to repair fabrics showing said streaks depending on the importance of the defect and the color of the dyeing. This operation, however, adds costs and increases the complexity of the supply chain logistics.

The problem at the root of the present invention is therefore to provide a simple, cost effective solution enabling to avoid or at least minimize the above mentioned problems.

SUMMARY OF THE INVENTION

This invention relates to a method for providing a dyed meta-aramid material, wherein said method comprises at least a step of pretreatment of the material and a subsequent step of dyeing the material, characterized in that the step of pretreatment is carried out under closed conditions, in the presence of an aqueous medium, and at a target temperature of 80 to 130° C. This invention also relates to fibers, yarns, and fabrics obtained from this method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical curve obtained with the x-ray diffractometer along with lines used in the Crystallinity Index (CI) determination.

DETAILED DESCRIPTION OF THE INVENTION

The meta-aramid material used in accordance with the present invention can be any meta-aramid fiber, any yarn including such fiber and/or any fabric including such yarn. Preferably, the meta-aramid material are fibers of poly(meta-phenylene isophthalamide) (MPD-I). One such meta-aramid fiber is the Nomex® meta-aramid fiber available from E.I. du Pont de Nemours and Company of Wilmington, Del.

The step of pretreatment of the aramid material is carried out under closed conditions. As employed herein, the words “closed conditions” mean without material transfer from the system to the environment. This can be accomplished in a vessel or piece of equipment that the aramid material can be put into and then the vessel or piece of equipment sealed and pressurized, if desired, during the pretreatment of the aramid material. Further, as employed herein, the word “system” includes the vessel or equipment used to conduct the pretreatment, the aramid material, and the fluid(s) and/or gas(es) used in the pretreatment. The “environment” includes the area external to and surrounding the vessel or equipment. In some preferred embodiments, the pretreatment is conducted batch-wise. By way of one example, pretreatment under “closed conditions” can preferably be accomplished in an autoclave or other closed vessel. The step of pretreatment can be carried out under pressurized conditions that are dictated by the vapor pressure of the aqueous medium in which the pretreatment occurs.

Additional pressure may be created, if needed, by adding gases into the system. Advantageously, the step of pretreatment comprises at least (1) a pre-phase of heating the aqueous medium to a desired target temperature and (2) a main phase where the target temperature is maintained constant. The main phase of the step of pretreatment typically has a duration from about 30 minutes to about 60 minutes.

One of the main purposes of the step of pretreatment is to equalize the crystallinity index (“CI”) of the meta-aramid material, which crystallinity index (“CI”) contributes, among other things, to drastically reduce the streakiness phenomenon of the meta-aramid material. The crystallinity index (“CI”) parameter as defined in the present application is measured in accordance with the method provided herein under “Crystallinity Index Determination”. Moreover, the step of pretreatment contributes to correct defects or inequalities that may have been created during the batch production process of the meta-aramid material by, among others, equalizing its crystalline structure and molecular voids.

In some embodiments, the step of pretreatment is carried out until all the aramid material within the closed system has a substantially equal crystallinity index. In some embodiments, the crystallinity index of the meta-aramid material after the step of pretreatment is between about 10 and about 13, as measured in accordance with the method laid down below under “Crystallinity Index Determination”. Preferably, after the step of pretreatment, the crystallinity index is below 14, which is believed to provide a meta-aramid material that is still in an amorphous phase, which can encourage proper uniform dyeing.

The step of pretreatment may be carried out at the dyeing facility, integrated to the dyeing process, thus avoiding additional costs for energy, transfer of material and/or other logistics and manufacturing costs.

The step of pretreatment may be applied, with the same conditions of temperature and time, to the meta-aramid material in form of fiber, yarn or fabric.

In one embodiment, the method includes a step of scouring the meta-aramid material before it undergoes the step of pretreatment. The main objective of the step of scouring is to remove any impurities and dirt on or in the meta-aramid material before it is pretreated and dyed. The step of scouring can be done in a scouring bath. According to one preferred embodiment the step of scouring is carried out at a temperature from about 60 to about 90° C. The meta-aramid material is typically scoured in an aqueous medium at a pH from about 6 to about 10, during a period of time typically varying from 20 to 40 minutes. As scouring agents, one or more surfactants acting in the above mentioned pH range can be used, like for example Tinoventin JU®. Tinoventin JU® is believed to be alkylphenolethoxylate, a non-ionic detergent produced by Ciba and available from Kremer Pigmente GmBH & Co. KG in 88317 Aichstetten, Germany.

The step of scouring can be carried out separately or in the same bath of the step of pretreatment. In the latter case the overall method is strongly simplified and the entire phase before the step of dyeing can be made in a single batch. Substantive energy savings are also achieved through the integration of the two steps since the meta-aramid material to be scoured and pre-treated undergoes a single temperature program. Accordingly, in one preferred embodiment, the scouring/pretreatment bath will be first heated to a temperature of 60 to 90° C. and will remain at this temperature for a period of from 20 to 40 minutes before being further heated at a temperature of from 80 to 130° C. at which it will stay for a period of from 30 to 60 minutes.

After the optional step of scouring and the step of pretreatment, the meta-aramid material undergoes the step of dyeing. Any conventional dyeing process for meta-aramid materials can be used, like those described in Huntsman's Maxilon® for acrylic, cationic dyeable polyester, modacrylic and meta-aramid fibers (512004e from Huntsman) or DyStar's, the new face of Astrazon® (03.1025-00 from DyStar)

The present description provides exemplary embodiments and is not intended to limit the scope, applicability, or configuration of the claims. Rather, the present description will provide those skilled in the art with an enabling description for implementing the described embodiments, it being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the appended claims.

Test Methods

Crystallinity index determination. Crystallinity in meta-aramid fibers is one of the factors affecting their dyeability and the dyeability of yarns and fabrics including such fibers. X-ray diffraction is used as a primary tool for the determination of the crystallinity of such fibers.

Equipment and Sample Holder: An X-ray diffractometer is used in these measurements, specifically a Xpert Pro from Panalytical, Almelo, The Netherlands. The sample is prepared by mounting on a 32 mm Al fiber holder 1 thin layer of fiber and then placing the 32 mm holder with sample in the X'Pert sample changer holder. Typical data collection parameters are listed below. X'Pert Setup:

PIXcel detector, PHD 35/75, 1 degree scanning window

Use Auto-slits for Incident Beam and Diffracted Beam

Length=10 mm for incident and diffracted beam

Receiving Slit: 0.3 mm

Scan parameters:

Scan: 3-40 deg 0.1 step size

Count time: run longer than 1 hour to reduce noise (essential)

Data collection: Continuous Mode

Spin sample 2 sec/rev

Calibration: Standard diffraction calibration procedures as applied by the technical service of Panalytical, Almelo, The Netherlands based on LaB6 or equivalent procedures are sufficient for this method.

Procedure: FIG. 1 illustrates a typical curve obtained with the x-ray diffractometer. While this specific curve is representative of a sample after a dyeing process was completed, therefore it has more crystallinity; the calculation procedure can be used on material at any time during the process. The same procedure is used to determine the crystallinity index for samples, for example, after pretreatment and before dyeing. The vertical axis is the equatorial intensity for a fabric or staple fiber and comprises oriented and un-oriented crystalline and non-crystalline intensity components and the horizontal axis is scattering angle, 2θ, in degrees.

The meta-aramid fabric Crystallinity Index (CI) data analysis procedure is outlined below, as illustrated on FIG. 1.

-   -   Plot data 0 to 40 degrees as shown on FIG. 1; no corrections are         performed on the data as obtained directly from the         diffractometer     -   It is preferable to run for a sufficiently long enough time to         yield smooth data directly from the measurement in order to get         a steady state measurement with good stability.     -   A baseline is drawn from ˜3 to ˜36° in 2θ as shown in FIG. 1.     -   Draw a local baseline (DE) on FIG. 1. Draw a parallel line to DE         to touch the top of the 27° peak at a single point A.     -   Draw a vertical line passing through A, intersecting the local         baseline

(DE) at B and the global baseline (red line) at C.

-   -   The Crystallinity Index (CI) is calculated from the intensities         at A, B and C as follows,

CI={[intensity(A)−intensity(B)]/[intensity(A)−intensity(C)]}×100

The calculation on the specific curve shown in FIG. 1 gives the following result:

CI={A−B(150−100=50) divided by A−C(150−40=110)}×100=45

Examples

In the Examples, the step of scouring, both in the separate and integrated type of treatment, was carried out in the presence of Tinoventin JU® at the conditions listed in Table 1.

In Example 1, the step of scouring was carried out separately and prior to the step of pretreatment. The step of scouring was carried out in a AHIBA™ Turbomat from Datacolor Ahiba, Switzerland, headquartered in Lawrenceville, N.J., USA. The step of pretreatment was carried out in water only, at the conditions listed in Table 1.

All Examples 1, 2, and 3 were dyed according to the following procedure (color royal blue):

-   -   Set the dye bath with warm (30-40° C.) water and add         -   70 g/l Benzyl alcohol         -   1-2 g/l non-ionic dispersing agent     -   Run for 10 minutes and then add         -   1% Basic Blue 41 (dissolved in boiling water and acetic             acid)     -   Run for 10 minutes and then start to heat with 1.5° C./min     -   Add during the heating (between 40 and 80° C.) in 3 portions         -   20 g/l sodium nitrate     -   Adjust the pH to 4.0 with acetic acid     -   Heat with 1.5° C./min to 120° C. and hold the temperature 60         minutes     -   Cool down to 80° C., drop the bath and rinse with hot and cold         water     -   Set a new bath with 1 g/l non-ionic dispersing agent and adjust         pH to 5.0 with acetic acid     -   Run at 90° C. for 30 minutes and rinse once with hot and twice         with cold water

The dyeing uniformity was assessed by optical evaluation. The optical evaluation was made with naked eye to detect either darker, lighter streaks or uniform dyeing which is particular visible when using a royal blue color.

From a practical standpoint Examples 2 and 3 present the advantage, among others, to limit the operation to a single step since the step of scouring is integrated with the step of pretreatment and the crystallinity index (CI) homogenization is happening adequately.

As shown in Table 1, the step of pretreatment keeps the crystallinity index below 14. Consequently the meta-aramid material is still in an amorphous phase, which provides a material for proper uniform dyeing.

TABLE 1 Material Fabric 2/1 twill 220 g/m2 100% amorphous meta-aramid fibers Test Control 1 2 3 Material (g) n/a 10 10 10 Liquor (ml) n/a 150 150 150 M/L* (g/ml) n/a 1/15 1/15 1/15 Type of n/a Separate Integrated Integrated pretreatment Scouring/Pretreatment Scouring/Pretreatment Scouring/Pretreatment Standard n/a 1% non ionic n/a n/a scouring detergent washing ingredients agent and sodium carbonate pH n/a 9 n/a n/a Scouring n/a 80 n/a n/a temperature (° C.) Scouring n/a 30 n/a n/a time**** (min) Pretreatment n/a Water only 1% non ionic washing 1% non ionic washing ingredients detergent agent and detergent agent and sodium carbonate sodium carbonate pH n/a 7 9 9 Pretreatment n/a 130 130 130 temperature (° C.) Pretreatment n/a 60 60 30 Time*** (min) Resulting CI after pretreatment, prior dyeing process Crystallinity 7.3 10 13 13 index** (CI) Dyeing Non Excellent Excellent Excellent Uniformity acceptable after dyeing *Material/Liquor ratio **At the end of the step of pretreatment ***Calculated from the moment the liquor has reached the scouring or pretreatment temperature. 

1. A method for providing a dyed meta-aramid material, wherein said method comprises at least a step of pretreatment of the material and a subsequent step of dyeing the material, characterized in that the step of pretreatment is carried out under closed conditions; in the presence of an aqueous medium; and at a target temperature of 80 to 130° C.
 2. The method of claim 1, further comprising a step of scouring the material prior to the step of dyeing the material.
 3. The method of claim 2, wherein said step of pretreatment is made after the step of scouring.
 4. The method of claim 2, wherein the step of scouring and the step of pretreatment are combined in a same bath.
 5. The method of claim 2, wherein the step of scouring is carried out in an aqueous medium at a pH between of 6 to
 10. 6. The method of claim 1, wherein the material is a fiber, a yarn including said fiber, or a fabric including said yarn.
 7. The method of claim 1, wherein the step of pretreatment comprises a pre-phase of heating the aqueous medium to the target temperature and a main phase where the target temperature is maintained constant.
 8. The method of claim 7 wherein the duration of the main phase of the step of pretreatment is from 30 minutes to 60 minutes.
 9. A fiber obtained by the method of claim
 1. 10. A yarn obtained by the method of claim
 1. 11. A fabric obtained by the method of claim
 1. 